1. Field
Embodiments of the present invention relate to optics and, in particular, to image scanning.
2. Discussion of Related Art
Flexible endoscopes are commonly used in medical applications to look inside the human body to check the status of such organs as lungs, intestines, and colon, for example. Presently available flexible scope designs use either a bundle of optical fibers, typically in a tube, and/or one or more cameras having an array of detectors to capture an image.
Many commercially available endoscopes suffer from a fundamental tradeoff between high image quality and small size, however. For example, the diameter of currently available flexible endoscopes cannot be reduced to smaller than the image size. The diameter also is limited by the individual pixel size of a camera or by the diameter of optical fibers used to acquire the image. Currently, the smallest pixel element is determined by the size of the end of an optical fiber, which has a minimum core diameter of about four micrometers (4 μm). To propagate light through an optical fiber, a surrounding cladding layer is required. The cladding requirement increases the minimum pixel size to more than 5 μm in diameter. If a standard video graphics adapter (SVGA) image is desired, (e.g., with a resolution of 640×480 pixels), then a minimum diameter required for just the imaging optical fiber is more than three millimeters (3 mm). Larger diameters adversely affect the fineness of detail that can be distinguished in an image or resolution. Larger diameters also adversely affect the area that is visible through the endoscope or field of view (FOV). Therefore, to achieve endoscopes with less than 3 mm overall diameter using current technologies, resolution and/or field of view must be sacrificed by having fewer pixel elements.
Currently available endoscopes also suffer from poor control mechanisms. Some optical systems use an optical fiber and a charge coupled device (CCD) camera at a tip of the flexible fiber to illuminate a region of interest and acquire an image. The optical fiber and camera are manually controlled by a practitioner positioning the tip of the flexible fiber. Other optical systems use a resonant fiber that is actuated into resonance with one or more nodes to produce a desired illumination spot. Although these systems actuate the fiber, such systems cannot precisely control the position of the fiber tip without adding material to the fiber scan system and increasing the diameter and/or tip length.
Other optical systems deflect or move mirrors to position the light beam rather than move the waveguide. However, the mirrors must be larger than the light beam diameter to avoid clipping the beam or adding diffraction. Thus, the mirrors must be larger than the waveguide, thereby increasing the overall size of the instrument.
Some microscopes actuate a cantilever waveguide for near-field imaging. However, near-field systems have a very limited field of view (e.g., typically less than 500 nanometers), and a light-emitting tip must be positioned within nanometers of the target. Near-field systems are based on emitting light through a microscopic aperture with dimensions smaller than the wavelength of visible light. The emitted light reflects off the closely positioned target and is detected before the light has time to diffract and dissipate. A near-field system may be useful for imaging individual cells or molecules, but is not suitable for most medical procedures and other dynamic applications, which require a field of view of at least a micron and can not be dependant on precisely positioning a tip within nanometers of a target. Using larger wavelengths to provide a suitable field of view with a near-field system would still require a substantially larger imaging system, which could not be integrated into a multi-function instrument.
The concept of a micro-machined scanning optical microscope has also been explored in the form of confocal scanning microscope designs that employ a resonant XY bimorph stage, a resonant cantilever probe and lens, or at least one resonant micro-mirror. The confocal design for image acquisition has the advantage of spatially filtering the backscattered light while using the same optical fiber for illumination and signal collection. However, the extremely low efficiency of light collection (into the core diameter, typically few microns) of this design remains a disadvantage. Furthermore, confocal systems are limited to single wavelength operation, which does not enable color imaging or display.